Session protection system

1. A packet system operating in ATM, wherein each cell has a header to identify a connection or link, the system including a controller to ensure that rules are followed to ensure: i) ATM cells are delivered in the order transmitted, and ii) ATM cells are delivered with a consistent VP or VP/VC value said connection or link being either for a virtual path (VP), where cells are delivered with the same VP field but potentially different VC fields; or a virtual circuit (VC), where cells are delivered with the same VP and VC fields, the VP and VC fields being contained within the cell header and negotiated on a link by link basis but being static for a link, said ATM defining an adaptation layer for transmitting packets through a VC, whereby the adaptation layer ATM endpoints break packets into cells and the transmission of packets through the network relies on both of said rules for correct operation; the packet switching system further including a session protection system which disrupts the controller and employs a pseudo random number generator, located in a transmitting network element to break at least one of said rules by: (a) either delivering ATM cells in an order differing from that in which they were transmitted, (b) or delivering ATM cells with a non-consistent VP or VP/VC value, and which also employs a further pseudo random number generator located in a receiving network element either to reassemble the received ATM cells so that they are in the order of transmission, or so that they have a consistent VP or VP/VC value respectively, the session protection system providing said network elements with exchange keys to synchronise said pseudo random generators.

2. A packet switching system according to claim 1 , wherein session protection system breaks both of said rules.

3. A packet switching system according to claim 1 , wherein parameters for controlling the pseudorandom number generator include any one or more of (a) the number of cells over which to re-order, (b) maximum packet size expected, (b) a connections delay variation (CDV).

4. A packet switching system according to any preceding claim, wherein there is only one active VC, and where a dummy stream of cells is generated to interleave with the real stream of cells.

5. A packet switching system according to any preceding claim, wherein errors can occur that cause cells to become corrupted, and with regard to rule (i), the receiver network element marks bad cells to be kept until re-assembly is required; and with regard to rule (ii) the receiver element notes when cells arrive, whether cells are transmitted across the link at a fixed rate; cells being treated as null cells if they do not contain data from an active VC.

6. A packet switching system according to an preceding claim, wherein said adaptation layer is one of ATM layers AAL 1 , AAL 2 or AAL 5 .